Mirrored armature animations
Just rig one half of the model and mirror the mesh after the Armature modifier.

If you need to mirror pose bone poses, you can use the Copy/Paste buffers to copy a mirrored pose to the other side (Ctrl C, Shift Ctrl V).

If you need one side of the bones to follow the other side interactively, you will have to use constraints.

The Copy Location/Rotation/Scale constraints should do the work. Setup them like this (for mirroring on X axis):



Note that you will be able to only pose one half of the model with the other just being a "dummy side", else there would be dependency issues.